This is one of the Level 3 core courses to students pursuing a statistics major.
Among the three cores, I personally believe this is the hardest of the lot. MATH3821 gets its difficulty from having too much content. This course is about probability theory, and that's not something you can simply overcome with memorisation.
This course is pretty popular among mathematics students as a whole due to the fact that MATH2931 is not a prerequisite and that it's really just a probability course, not statistics. Part of this is influenced by the fact that many people genuinely enjoy the first half of MATH2901, and drop off in the second. Mastery of this course can be extremely useful in the short term, because the strengthened intuition in probability can help useful in trading interviews, where probability based questions resurface more than you might expect.
Stochastic processes are just useful in general for people who want to work with the market. A simple summary: a stochastic process is just a sequence of random variables measured over time (can be both discrete time and continuous time).
Assessment tasks are definitely more challenging in this course, compared to its sister course MATH3911. Although most questions are doable, every past paper I've seen seems to introduce a completely unrelated question to that of the previous paper. They all use the same techniques, but the question style appears foreign.
For the class tests, the questions are the same. The difference is that he changes the letters in the actual test. (Or for MATH3801, he might change a number instead.) Figure out a way to write it as quickly as you can, but not too quickly that you'd struggle with different letters/numbers. Some explanation skipping is okay, just not too much.
Also, after studying for the finals, I would advise being careful with Brownian motion. It's also a really interesting concept to learn about, but it can take a while to understand because it's so far to the end and is fairly different to the stochastic processes you see before it.
(The difficulty would've been 4/5, but it jumped to 4.5/5 after I saw the challenge of this year's midsession. It was a pretty spiked paper to offset that we had all the lecture notes available, and not just the cheat sheet.) Edit: I'm convinced 4.5/5 was the right choice now after studying for finals.