What does 'signal' and 'lock-in' mean in a BIP? I have a vague idea about the jargon used over there, but what exactly do 'signal' and 'lock-in' mean specifically?
Signalling simply means the miner of a block has set a bit in the version field to say that they support something. BIP 8 and 9 discuss this, it allows the miners to let the network know they are ready for the change or not. The version field of a block is 32 bits long, and if the top 3 bits are set to '001', 29 bits are free to be used for signalling. 

After a certain start time defined for a particular proposal, miners can set the bit specified to signal their support. It is during this time that if at least a certain percentage of blocks are mined with the bit set in one difficulty retarget period (2016 blocks), specified as the threshold needed, that proposal becomes LOCKED_IN. Once it is locked in, it is ready to become active in the network. Locked in just means that it gained enough support to proceed.

This can all be found in BIP 9.