The #MSWL stands for Manuscript Wishlist. It's a searchable tag on Twitter. Agents, editors, and even small presses use it to tell you, the author, about their manuscript wishlists.
And that would be enough, but there's more! There is an entire website dedicated to MSWL, Conveniently called ManuscriptWishlist.com where you can search the website. You can put in subjects, for example "romance" and a list of agents who represent, and are looking for romance novels comes up.
There are specific days on Twitter when agents will post their #MSWL and the tag can be searched at any time. But, they may not be the most up to date.
Another tags you should be aware of is this one:
#ownvoices- this means that the agent or editor is looking for diverse work written by those from the background that they are utilising in the book. So, LGBTQ books written by LGBTQ people, and this can apply to ethnicity, gender, whether or not the author is able-bodied, etc.
Other than that, the usual genre tags will apply. If you don't like Twitter, just cruise the website and see if you discover a new agent or publisher for your work!