die Vertiefung
depression / hollow
A place that is lower than its surroundings; a hollow or cavity in a surface.
Nach dem Regen sammelte sich Wasser in der Vertiefung im Weg.
After the rain, water collected in the depression in the path.

depression / deepening
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depression / hollow
A place that is lower than its surroundings; a hollow or cavity in a surface.
Nach dem Regen sammelte sich Wasser in der Vertiefung im Weg.
After the rain, water collected in the depression in the path.

'Vertiefung' (depression/deepening) comes from the verb 'vertiefen' (to deepen), which is made up of 'ver-' and 'tief' (deep). It describes the process of making something deeper—whether a pit in the ground or one's understanding of a topic.
Durch die Lektüre des Fachbuchs erreichte er eine Vertiefung seines Wissens.
By reading the specialist book, he achieved a deepening of his knowledge.
The context makes the meaning clear. If it is about surfaces, floors, or walls, a physical hollow is meant. If it is about knowledge, topics, or learning, intensive engagement is meant. With friendship or relationships, it is about their intensification.
Achtung, da ist eine tiefe Vertiefung in der Straße!
Careful, there is a deep depression in the road!
In the context of learning, 'Vertiefung' (deepening) is often used with genitive objects: 'die Vertiefung des Wissens' (the deepening of knowledge), 'die Vertiefung eines Themas' (the in-depth study of a topic), or 'zur Vertiefung der Inhalte' (to deepen the content).
Die Hausaufgaben dienen der Vertiefung des Gelernten.
The homework serves to reinforce what has been learned.
When it is about relationships or feelings, 'Vertiefung' (deepening) describes a positive intensification. Typical combinations are 'die Vertiefung der Freundschaft' (the deepening of the friendship), 'der Zusammenarbeit' (of the collaboration), or 'der Beziehungen' (of the relationships).
Die Reise trug zur Vertiefung ihrer Freundschaft bei.
The trip contributed to the deepening of their friendship.
impression
'der Eindruck' and 'die Vertiefung' can both refer to a deeper spot in a surface. 'Eindruck' emphasizes that this spot was made by pressure, for example by a finger or a tool. 'Vertiefung' mainly describes the shape: one spot is lower than the area around it. How it was created is not always important. If you want to mention the cause as pressure, 'Eindruck' is more precise. If only the deep spot itself is important, 'Vertiefung' is more natural.

In der Wand gibt es eine kleine Vertiefung, in die das Regal genau passt.
There is a small recess in the wall that the shelf fits into exactly.
Auf dem Kissen war noch der Eindruck von ihrem Kopf zu sehen.
The impression of her head was still visible on the pillow.