What is the spell Alohomora?

It´s the spell to open closed doors

Alohomora is a spell to open locks on doors and so on.

A basic charm learned by first years use to unlock doors and so on

to unlock a locked door as seen in the Sorcerers stone used by Hermione.

It unocks doors. It's pretty much only useful with Muggle stuff, though, because wizards usually use magic to secure their doors.