getblocktemplate ( "jsonrequestobject" )

If the request parameters include a 'mode' key, that is used to explicitly select between the default 'template' request or a 'proposal'.
It returns data needed to construct a block to work on.
For full specification, see BIPs 22 and 9:

1. "jsonrequestobject" (string, optional) A json object in the following spec
"mode":"template" (string, optional) This must be set to "template" or omitted
"capabilities":[ (array, optional) A list of strings
"support" (string) client side supported feature, 'longpoll', 'coinbasetxn', 'coinbasevalue', 'proposal', 'serverlist', 'workid'

"version" : n, (numeric) The block version
"rules" : [ "rulename", ... ], (array of strings) specific block rules that are to be enforced
"vbavailable" : { (json object) set of pending, supported versionbit (BIP 9) softfork deployments
"rulename" : bitnumber (numeric) identifies the bit number as indicating acceptance and readiness for the named softfork rule
"vbrequired" : n, (numeric) bit mask of versionbits the server requires set in submissions
"previousblockhash" : "xxxx", (string) The hash of current highest block
"transactions" : [ (array) contents of non-coinbase transactions that should be included in the next block
"data" : "xxxx", (string) transaction data encoded in hexadecimal (byte-for-byte)
"txid" : "xxxx", (string) transaction id encoded in little-endian hexadecimal
"hash" : "xxxx", (string) hash encoded in little-endian hexadecimal (including witness data)
"depends" : [ (array) array of numbers
n (numeric) transactions before this one (by 1-based index in 'transactions' list) that must be present in the final block if this one is
"fee": n, (numeric) difference in value between transaction inputs and outputs (in Satoshis); for coinbase transactions, this is a negative Number of the total collected block fees (ie, not including the block subsidy); if key is not present, fee is unknown and clients MUST NOT assume there isn't one
"sigops" : n, (numeric) total SigOps cost, as counted for purposes of block limits; if key is not present, sigop cost is unknown and clients MUST NOT assume it is zero
"weight" : n, (numeric) total transaction weight, as counted for purposes of block limits
"required" : true|false (boolean) if provided and true, this transaction must be in the final block
"coinbaseaux" : { (json object) data that should be included in the coinbase's scriptSig content
"flags" : "flags" (string)
"coinbasevalue" : n, (numeric) maximum allowable input to coinbase transaction, including the generation award and transaction fees (in Satoshis)
"coinbasetxn" : { ... }, (json object) information for coinbase transaction
"target" : "xxxx", (string) The hash target
"mintime" : xxx, (numeric) The minimum timestamp appropriate for next block time in seconds since epoch (Jan 1 1970 GMT)
"mutable" : [ (array of string) list of ways the block template may be changed
"value" (string) A way the block template may be changed, e.g. 'time', 'transactions', 'prevblock'
"noncerange" : "00000000ffffffff", (string) A range of valid nonces
"sigoplimit" : n, (numeric) cost limit of sigops in blocks
"sizelimit" : n, (numeric) limit of block size
"weightlimit" : n, (numeric) limit of block weight
"curtime" : ttt, (numeric) current timestamp in seconds since epoch (Jan 1 1970 GMT)
"bits" : "xxx", (string) compressed target of next block
"height" : n (numeric) The height of the next block

> bitcoin-cli getblocktemplate
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "getblocktemplate", "params": [] }' -H 'content-type: text/plain;'
Don't trust. Verify. Docs extracted from src/rpc/mining.cpp#L317-L388.