What we call "belly-dancing" (la danse orientale).

You asked for context; the context will make it clear when we have accurate subtitles.

– C’est bien jolie, la danse du ventre. Mais tu as fait une belle bêtise quand tu étais là-bas.

– Moi ?

– Je dis une bêtise, mais c’est presque un crime.

– Je ne vois pas de quelle bêtise tu parles. Quand j’étais blessé, ils allaient me nommer caporal.

– Ça c’est autre chose. Je pense à une lettre que tu as reçue.

– Quelle lettre ?

– Une qui méritait bien une réponse. Et toi tu n’as pas répondu.

– Une lettre de qui ?

They’re talking about his youth and his service abroad ("là-bas"). While there, a letter from his true love was addressed to him telling him she was pregnant with their child. But César never answered her letter. Delphine chastises him for this, supposing that he forgot his true love back home in favour of the local beauties with their sex appeal — "la danse du ventre". In his defence he says he never received the letter. And we believe him, because if he had read it, he would have known

that he was orchestrating his own son’s death.

XouDo in the comments cites an interesting figurative meaning, perhaps equivalent to our "wave a magic wand" or something of the kind.

The full sentence is:

C’est bien joli la danse du ventre mais tu as fait une belle bêtise quand tu étais là-bas.

Belly-dancing is nice to see, but you made a big mistake when you were there.

With belly-dancing, Delphine refers to the time César spent in Northern Africa as a soldier when a young man. She implies that César was more interested in the nightlife there than in what he had left behind at home, especially by ignoring a letter sent by Florette to him when he was there.

Later, in the movie, we understand that she is wrong, and that the letter was unfortunately lost. César, who was in love with Florette before leaving for the army, mistakenly thought that it was not reciprocal, and Florette, because of the lack of a reply, came to the same conclusion.

I’m not sure about what precedes the scene in which this line is said in the movie but reading the book from where the movie is based allows to understand why Delphine talks about belly-dancing.

  Un soir d’automne, ils bavardaient comme d’habitude, en tournant le dos au soleil couchant dont les derniers rayons leur chauffaient les épaules.
  Le Papet parlait de l’Afrique, des moutons rôtis entiers dans un trou, des petites danseuses arabes qui faisaient la danse du ventre, au son des flûtes et des tambourins. La vieille l’écoutait sans mot dire, puis elle tourne vers lui ses yeux morts, et dit :
  « C’est bien joli, la danse du ventre, mais tu as fait une belle bêtise […]

Manon des sources, Marcel Pagnol, 1963.