Rattachisme (or Reunionism) is a part of the Walloon Movement that advocates the unification of Wallonia (and maybe Brussels and the six Flemish municipalities with language facilities for French-speakers) with France.
The main rattachist political parties are the Walloon Rally (partially) and the Wallonia-France Rally. They still have not gained any parliament seats so far.
The movement for Belgian or Walloon union with France has existed since Belgium was created.
The Southern Netherlands were invaded and annexed by the First French Republic in 1795, ending Habsburg rule. After Napoleon's defeat at Waterloo in 1815, the major victorious powers agreed at the Congress of Vienna on reuniting the former Austrian Netherlands and the former Dutch Republic, creating the United Kingdom of the Netherlands, which was to serve as a buffer state against any future French invasions. King William imposed Dutch as main language, while French was the language of the bourgeoisie in the Southern Netherlands. They were also significantly underrepresented in the Dutch Assembly. These and other reasons caused in 1830 the Belgian Revolution, supported by France. The Southern Netherlands declared independence, becoming the Kingdom of Belgium. However, a lot of the proponents of secession from the Netherlands, preferred union with France rather than an independent country, but the major European powers decided on an independent neutral Belgium in the Treaty of London.
The "r" in "rattachism" (from "re-" and "attach") indicates that the movement is to re-unite with France. However, Belgium/Wallonia was only together with France for about 20 years.