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 par first years use to unlock doors and so on

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

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