FOR UPDATE keyword in SOQL helps us to lock the records from being updating from another request.

Sample Code:

List<Contact> listCon = [SELECT Id, FirstName FROM Contact WHERE FirstName LIKE ‘%test%’ FOR UPDATE];


for(Contact con: listCon) {
    …………………
    …………………
}

Things to Know:
1.  If you attempt to lock a record currently locked by another client, your process waits for the lock to be released before acquiring a new lock. If the lock isn’t released within 10 seconds, you will get a QueryException. 
2. If you attempt to update a record currently locked by another client and the lock isn’t released within 10 seconds, you will get a DmlException.
3.  The record locks that are obtained in Apex via FOR UPDATE clause are automatically released when making callouts.