First Two Blocks, or F2B, is a block building method, most notably used as a first and second step in Roux. It involves building two 1x2x3 blocks(on a 3x3x3), on opposite sides of the cube.
This can be done in a variety of ways though the most common way is to build the first 1x2x3 in DL followed by a second 1x2x3 in DR. However, it is also possible to build them simultaneously (though this impedes lookahead) or by placing the DL and DR edges then solving in a way similar to CFOP F2L with freedom of the M-Slice. However, this is usually seen as less efficient as well while not helping much in the way of lookahead or tps.
It has been proposed that F2B could also be solved at the same time as other parts of the cube such as EO or CP (the latter being the first 2 steps of the B2 Method). However this has never caught on as the addition can add considerable strain to the users inspection time and limits the lookahead.
In the early days of the term, it was also used to refer to ZZF2L or the second step in the ZZ Method. However, this has fallen out of favour in modern time where the term is almost exclusively used when discussing blocks which resemble the Roux method blocks