locker
relaxed
Relaxed, informal, and not formal in behavior or atmosphere.
Er hat eine sehr lockere Art und kommt mit jedem gut aus.
He has a very relaxed manner and gets along well with everyone.

loose / relaxed
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relaxed
Relaxed, informal, and not formal in behavior or atmosphere.
Er hat eine sehr lockere Art und kommt mit jedem gut aus.
He has a very relaxed manner and gets along well with everyone.

The word 'locker' describes a state that is not firm, stiff, or tense. This idea appears in all meanings: with objects, in social situations, and also with tasks.
Nach der Massage fühlten sich meine Muskeln wieder ganz locker an.
After the massage, my muscles felt completely loose again.
In colloquial speech, 'locker' (easily) is often used as an adverb in the sense of 'easy' or 'effortlessly'. It emphasizes that a task or action was handled without much effort.
Den Marathon schafft er dieses Jahr locker unter vier Stunden.
He'll easily manage the marathon in under four hours this year.
In the sense of 'entspannt' (relaxed), 'locker' often describes people or the mood. Typical phrases are 'eine lockere Atmosphäre' (a relaxed atmosphere), 'ein lockeres Gespräch' (a casual conversation), or 'nimm es locker!' (take it easy!).
Auf der Party herrschte eine sehr lockere und freundliche Atmosphäre.
There was a very relaxed and friendly atmosphere at the party.
When something is physically not tight or firm, 'locker' (loose) is the right word. Common combinations are 'eine lockere Schraube' (a loose screw), 'ein lockerer Zahn' (a loose tooth), or 'lockere Kleidung' (loose clothing).
Mein kleiner Bruder hat seinen ersten lockeren Zahn.
My little brother has his first loose tooth.
To describe an airy and not firm consistency, 'locker' (light/fluffy/loose) is often used in cooking, baking, or gardening. Typical examples are 'lockerer Teig' (light/fluffy dough) or 'lockere Erde' (loose soil).
Für ein gutes Wachstum brauchen die Pflanzen lockere Erde.
For good growth, the plants need loose soil.
simple / just
'einfach' and 'locker' can both mean that something does not take much effort. 'einfach' is neutral and mainly says that something is not complicated. In this meaning, 'locker' is more colloquial and often sounds as if something can be done very easily or without stress. Learners may confuse the words because 'locker' also has other meanings, for example relaxed or not tight. If you want to state objectively that a task is easy, 'einfach' fits better. 'locker' sounds more natural in casual everyday speech.

Tom hat die Prüfung locker geschafft, obwohl er vorher nervös war.
Tom passed the exam easily, even though he had been nervous beforehand.
Die Anleitung ist einfach, deshalb kann ich den Schrank allein aufbauen.
The instructions are easy, so I can assemble the cabinet by myself.