Here is a Scenario:
PF: Flaps one.
PM: Speed Checked, Flaps one.
What does that mean and why do they have to say "speed checked" every time?
Because there is a maximum deployment speed for things like flaps, gear extension (and sometimes retraction). Exceeding these speeds and using those devices can cause damage to control surfaces or other equipment. The speed check is there so that when the PF calls for flaps, the PM remembers to check the speed before performing the action.
Some companies mandate that the PM verbalise "Speed checked", but it's utterly useless verbiage.
Firstly, the PF should already have checked the speed before asking for the next flap setting.
Secondly, pilots have a history of simply saying "Speed checked", but not actually checking anything. All part of the ritual of PF says Flaps x, I say "Speed checked" and move the Flap Lever.