If some resources don't work full-time for the company, you may want to enter the agreed part-time schedule in the User edit panel.
To do so, go to the User section, select the user and choose the tab Schedule
In the above example, this user is working regular hours on Monday, Tuesday and Friday; on Wednesday s/he is only available half day and not at all on Thursday. Note that the lunch break hours will also be represented so to automatically position the schedule accordingly.
The planner board will correspondingly show this number of hours on the specific days and as well grey out the slot this person is not available.
Alternatively, this setting can be performed by the concerned user in his/her own User's profile. To do so, click on the User's Name (top right corner) and then select Profile.
The daily parameters can then be entered in the Schedule section:
Last, especially for resources having irregular working times, you may simply book their unavailability by booking these slots as time-off.
Let's say you have a resource that's not available certain Tuesdays: simply create a project "Not available" (non-billable!) and book the appropriate days as per this resource schedule. If applicable, you can also make use of recurring bookings (say "every month").