der Marktplatz
market square
A central public square in a town or village where markets take place regularly.
Jeden Samstagmorgen kaufe ich frisches Gemüse auf dem Marktplatz.
Every Saturday morning I buy fresh vegetables at the market square.

market square / marketplace
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market square
A central public square in a town or village where markets take place regularly.
Jeden Samstagmorgen kaufe ich frisches Gemüse auf dem Marktplatz.
Every Saturday morning I buy fresh vegetables at the market square.

'Marktplatz' is a compound noun made up of 'der Markt' (the market) and 'der Platz' (the square/place). So it literally describes a square where a market takes place. This logic helps you understand the basic meaning.
Das alte Rathaus steht direkt am Marktplatz.
The old town hall is located directly on the market square.
You can tell from the context whether the physical or virtual place is meant. Words like 'online', 'digital', 'im Internet' (on the internet), or names of platforms point to the digital meaning. Words like 'Stadt' (town/city), 'Dorf' (village), 'Brunnen' (fountain), or 'Rathaus' (town hall) point to the real square.
Auf dem zentralen Marktplatz der Stadt findet ein Fest statt.
A festival is taking place on the central market square of the city.
For the real market square, the prepositions are important. You are 'auf dem Marktplatz' (on/at the market square; dative, location), and you go 'zum Marktplatz' (to the market square; dative, direction) or 'auf den Marktplatz' (onto the market square; accusative, direction with entering the area).
Wir treffen uns um 14 Uhr auf dem Marktplatz vor dem Brunnen.
We'll meet at 2 p.m. on the market square in front of the fountain.
place / space
'der Platz' and 'der Marktplatz' can both mean an open place in a town or village. 'Platz' is more general: it can be any open public square between buildings. 'Marktplatz' is more specific and means a central square where markets take place or used to take place. The words can be confused because a 'Marktplatz' is also a 'Platz'. But not every 'Platz' is a 'Marktplatz'. If the market is important, 'Marktplatz' is more precise.

Am Samstag kaufen wir frisches Gemüse auf dem Marktplatz.
On Saturday, we buy fresh vegetables at the market square.
Auf dem großen Platz vor dem Rathaus sitzen viele Menschen in der Sonne.
Many people are sitting in the sun on the large square in front of the town hall.