der Tresen
bar
A long, high counter in a bar, café, or pub where drinks and food are ordered and served.
Er lehnte am Tresen und wartete auf sein Bier.
He leaned against the bar and waited for his beer.

counter / bar
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bar
A long, high counter in a bar, café, or pub where drinks and food are ordered and served.
Er lehnte am Tresen und wartete auf sein Bier.
He leaned against the bar and waited for his beer.

The position in relation to the counter is important. 'Am Tresen' (at the counter/bar) describes the position of the guest or customer in front of it. 'Hinter dem Tresen' (behind the counter/bar) describes the position of the staff serving people.
Zwei Gäste saßen am Tresen und unterhielten sich.
Two guests were sitting at the bar and chatting.
'Tresen' (counter/bar) is often used as the base word in compounds to describe the function more precisely. Typical examples are 'Bartresen' (bar counter), 'Empfangstresen' (reception counter), or 'Verkaufstresen' (sales counter).
Melden Sie sich bitte am Empfangstresen an.
Please check in at the reception counter.
In a bar or pub, 'Tresen' (bar/counter) is often combined with verbs like 'sitzen' (to sit), 'stehen' (to stand), 'lehnen' (to lean), or 'bestellen' (to order).
Komm, wir setzen uns direkt an den Tresen!
Come on, let's sit right at the bar!
table
'der Tisch' and 'der Tresen' can both be a surface where you stand, sit, or put something down. 'Tisch' is the general term for an everyday piece of furniture. 'Tresen' means a long, high counter, especially in a bar, café, pub, or at a reception desk. Learners may confuse the words because a counter can seem like a special kind of table. For eating at home or in a restaurant, you say 'Tisch'. When you order, pay, or are served, 'Tresen' fits better.

Am Tresen bestellt Max zwei Kaffee und bezahlt sofort.
At the counter, Max orders two coffees and pays right away.
Der Tisch am Fenster ist frei, aber der Tisch draußen ist schon reserviert.
The table by the window is free, but the table outside is already reserved.