tempo change line

Easing method
Normal: a linear transition effect with the same rate of change from start to end
Ease in: a transition effect with a slow change rate at the start but a quicker change rate at the end
Ease out: a transition effect with a quick change rate at the start but a slow change rate at the end

[EDIT] Definitions added to Handbook:
https://musescore.org/en/handbook/4/tempo-markings#Tempo_change_lines

> What is the ease in and ease out?
I am German and any translation of this sentence makes no sense to me ...
[Edit]: ok, in the meantime DanielR explained it - thank you!

Musescore offers several settings for the tempo. The most important are those of the tempo palette [MuS 3.6.2]:

All of these tempo settings can be applied to a note or rest, usually the very first one in the score, but also in the middle of a score if you have a tempo change.
For tempo settings with a number, you can edit and modify it yourself. Double-click it and edit the number.
The settings like Andante, Moderato, etc. have predefined values for tempo.
All those can be overritten in the properties if you uncheck "Follow written tempo" and specify a own number below - as shown in your second picture.

Then there are lines for tempo changes: rall., accel. etc., where you can specify a smooth change of tempo. This is what you can see in your first picture.

And finally, for exercises, for example, there is a temporary option to speed up or slow down the playback. This is not saved in the file and can be found in the playback controls at the top right of the MuS screen.