A Y cable is exactly what it says. It has one 1/4" plug on one end (typically plugs into the guitar), and it splits at the other end into two 1/4" plugs, to plug into two different things. The advantage is you can send one guitar output to two different places. The only possible disadvantage, depending on the quality of the cable, can be that you are literally splitting the signal in half, so it is a weaker signal going to each side of the Y. Another way to get the advantage, without the downside, is to use a pedal. There are plenty A-B-Y pedals. You plug your guitar cable into it, and it has two outputs (A and B). You can select to send the signal to A, B or to both (A+B or basically Y). Virtually all quality pedals don't split the level but send full signal to both outputs.
Good luck.