From July 3rd - 7th, the Maple Leafs are holding their annual Development Camp. It will include a bunch of practices, development work with skills coaches, and usually one exhibition game at the very end. Here's a write up I did on last year's Dev Camp, for reference:

Standouts from Toronto’s Development Camp
On Saturday, Toronto wrapped up it’s development camp with a scrimmage. Read up on who was said to have strong camps, and who looked good at the scrimmage.
PPP LeafsBrian S

The roster is always a mix of prospects they have drafted and signed, as well as a bunch of undrafted and unsigned guys that are invited. It's good for the players to work directly with an NHL team and get useful feedback on what to work on, and in Toronto's case they get an up and close look on prospects they may want to sign or draft in the future.

Sean Durzi and Mac Hollowell are two examples of players who went undrafted, attended Toronto's camp, and were ultimately drafted. Braeden Kressler is an example of a guy who went through the same process, but was signed to an ELC outright.

Here is this year's roster:

I'll have a bigger write up on all the invites, to see which of them may be interesting prospects for Toronto to look at.

Here's the write up that goes with the tweet:

Players will undergo medicals on Tuesday, July 2, while the first full-team on-ice session and media availability will take place on Wednesday, July 3, at Ford Performance Centre.
A total of 46 players (27 forwards, 15 defencemen, four goaltenders) are slated to take part in this year’s development camp. Attending this year’s camp will be 17 Maple Leaf draft picks, including six of Toronto’s selections from the 2024 NHL Draft. They include forwards Miroslav Holinka and Sam McCue, and defencemen Ben Danford, Victor Johansson, Matthew Lahey, and Nathan Mayes. Also in attendance will be three draft picks from the 2023 NHL Draft (Easton Cowan, Hudson Malinoski, Noah Chadwick), four draft picks from the 2022 NHL Draft (Nikita Grebenkin, Brandon Lisowsky, Fraser Minten, Nicholas Moldenhauer), one from the 2021 NHL Draft (Vyacheslav Peksa) and three from the 2020 NHL Draft (Artur Akhtyamov, John Fusco, Joe Miller).
In addition to the Maple Leafs’ draft picks in attendance, the roster will include three players signed to an NHL contract (Braeden Kressler, Jacob Quillan, Cade Webber) and five players signed to AHL contracts (Matthew Barbolini, Jacob Bengtsson, Chas Sharpe, Marko Sikic, Sam Stevens). Additionally, the Maple Leafs have a total of 22 free agents attending this year’s camp.
Maple Leafs Assistant General Manager, Player Development, Hayley Wickenheiser and her staff will oversee this year’s camp.

You can download the roster at this link.

Maple Leafs Prospects