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So I'm using this transfer orbit calculator and when simulating a transfer from LEO to Mars, in the delta-v breakdown, appear two different values: TMI (Trans-Mars Injection), at around 3000 m/s, and MTO (Mars Transfer Orbit), at around 600 m/s.
What is the difference between these two?
Confusing wording, in my opinion. See also e.g. Wikipedia:
A trans-Mars injection (TMI) is a heliocentric orbit in which a propulsive maneuver is used to set a spacecraft on a trajectory, also known as Mars transfer orbit, which will place it as far as Mars orbit. [emphasis added]
However, the numbers you give agree with this answer analysing the same trajectory. This leads me to conclude that the "TMI $\Delta v$" is for the transfer injection burn (leaving LEO into an elliptical trajectory intersecting Mars' orbit) while the "MTO $\Delta v$" is the burn after arriving at Mars in order to enter Mars orbit.
