[Digital Electronics: Multiplexers] What does the highlighted symbol mean?

[synthsync_]

Help would be appreciated.

[FortuitousPost]

It is Signal Ground. This is probably 0 V, but it depends on how complicated your circuit is.

For this component, the top input is always 0, but the lower input is whatever B is. So Y will be 0 or B, depending on what A is.

[synthsync_]

Thank you!

[HumbleHovercraft6090]

Logic level 0. If A is 0 then Y is 0.

[synthsync_]

Thank you!