Please attach the hose firmly to the part that sticks out underneath.
Water will come out when you turn on the tap.
「ホースを下にある凸部にしっかり嵌めて下さい!」 =
Please attach the hose firmly to the part that sticks out underneath.
嵌める = insert, put in, attach
下にある凸部 = the part that sticks out underneath; the convex part below
蛇口を回すと水が出る = water will come out when you turn on the tap.