The Bitcoin transaction lock time is the time at which a particular transaction can be added to the blockchain. This is the earliest time that miners can include the transaction in their hashing of the Merkle root to attach it in the latest block to the blockchain.
There are two specific types of transaction locktime. Firstly when the locktime figure is less than 500 million it is interpreted as a blockheight and miners therefore have to wait until that block height has been reached before attempting to include it in a block.
If it is above 500 million it is converted to a unix timestamp – a unix timestamp being the number of seconds since January 1st 1970.