With using locktime feature, you can make a transaction which can't be mined until a certain time/block number. But it doesn't prevent you from spending the fund earlier.
This is different to what is being offered by Coinb.in as quoted by Plaguedeath. It utilizes the OP_CHECKLOCKTIMEVERIFY OP code in the redeem script, rather than the nLockTime field in the transaction data itself.
Let's take the following public key as an example, which is generated from the private key 0000....0001:
0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798
Let's also say we don't want any coins on this address to be spendable until block 800,000.
So we head to
https://coinb.in/#newTimeLocked and paste in our public key, select "blockheight", enter 800000, and hit "Submit". We then get shown a P2SH address which has the following redeem script:
0300350cb175210279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798ac
Let's break that down:
03 - push 3 bytes to the stack
00350c - little endian encoding of the number 800,000
b1 - OP_CHECKLOCKTIMEVERIFY
75 - OP_DROP
21 - push 33 bytes to the stack
The next 33 bytes are the public key we pasted in as above, followed by 0xac, which is OP_CHECKSIG.
So this script essentially checks if we have reached the necessary block (or Unix time) specified. If we haven't, it terminates in an error. If we have reached the necessary height/time, then OP_CHECKLOCKTIMEVERIFY will verify, OP_DROP will clear the stack, and then all that is left will be the pubkey and OP_CHECKSIG as it would be in a old school P2PK output.