Usually, a resource should be assigned to a task at a level of 1.0, if that's the only task you want the resource to be scheduled on during any particular period of time. Or, if the resource should be able to split his/her time to work on two tasks simultaneously, assign the resource to each task at no higher than .5; for the resource to be able to work on three tasks simultaneously, assign the resource at no higher than .33 on each task, and so forth. Whenever the schedule is recalculated, note whether or not Level Resources is checked (Options, Calculations).
The Max Available field for each resource is edittable in the resource spreadsheet (Window, Resource). This number is usually set to 1.0, which indicates to Time Line that a maximum of 1.0 of this resource's time is available to work on tasks in the schedule. That will be the maximum workload that Time Line will schedule for that resource (when Level Resources is checked), which you can easily prove by looking at that resource's histogram. Note that changing the value for Max Available does NOT change any of the levels of that resource assigned to tasks. Make sure you understand how the Level Resources feature works before you make very many resource assignments.