The phrase "liquid glue" refers to a type of adhesive that is in liquid form, which can be easily spread and used to stick things together. Full definition
Using liquid glue and / or nails place two strips on both sides (on back). (iheartnaptime.net)
We glued them together with glue sticks because a) I hate the mess of liquid glue and b) we didn't have any other kind of glue in the house. (therhouse.com)
Ours were pretty sturdy using the glue sticks (after the glue dried), but I think liquid glue would increase stability. (therhouse.com)