Marx means that Bakunin's writings are mostly important to intrigue, or draw in, people. According to Marx, the purpose of history is to change the world. Marx goes further than retelling history and devises (or tries to devise) a path forward for society. This path is essentially communism. So basically Bakunin's ideas on anarchism are intriguing for Marx but they do not offer any way to move forward. This is the critique Marx has of philosophers. Philosophers merely ponder, they do not plan.
The line is from a letter to Friedrich Bolte, written in 1871, during the period of conflict between the factions around Marx and Bakunin in the International. All of the competing factions are fairly summarily dismissed, but the section on Bakunin is particularly dismissive, with the central charge being that:
For Mr. Bakunin the theory (the assembled rubbish he has scraped together from Proudhon, St. Simon, etc.) is a secondary affair – merely a means to his personal self-assertion. If he is a nonentity as a theoretician he is in his element as an intriguer.
There's not a lot of substance to the critique. You can get a more specific sense of what Marx thought by reading his "Marginal Notes on the Program and Rules of Bakunin's International Alliance of Socialist Democracy," but even there it is probably fair to say that Marx's marginal notes don't indicate a very engaged or even-handed reading of the document in question. From his side, of course, Bakunin was engaged in similarly unfriendly critique, as we can see in his correspondence and manuscripts.
Marx's claim that Bakunin was "in his element as an intriguer," depends on his belief that the International Alliance of Socialist Democracy was a conspiratorial attempt to control the International. It's a claim that would have to be weighed against a careful reading of the documents of the Alliance, the letters in which Bakunin explained his ideas about an "invisible conspiracy," and, of course, against the counter-claim that i[t] was Marx and his faction that were actually engaged in a conspiratorial bid for control through the General Council. Those waters get deep fast.
As for Bakunin's qualifications as a theorist, and his motivations for engaging in theory, we can now easily point to the period around 1870-71 as a period of extraordinary activity on his part, as he worked on several extensive, but ultimately unfinished works of anarchistic theory, including L'Empire knouto-germanique et la Révolution sociale (from which sections like "God and the State" were ultimately drawn.) Marx's accusations seem hard to square with Bakunin's life-long interest in philosophical questions and his continued attempts to complete various ambitious works, despite the distractions and pressures of his activism.