This is the 9378th most frequent Russian word.
тель
The word "тель" on its own does not have meaning in Russian; it may form part of a longer word or suffix (e.g., "-тель" meaning "-er" or "-or" in professions/actions).
In this context, 'тель' is a part of the word 'учитель', which means 'teacher', deriving from the root 'учить' (to teach), with 'тель' being a suffix that helps form a noun describing a person performing the action.
Учитель объяснил новый материал.
The teacher explained the new material.
Here, 'тель' is part of the word 'фильтр', referring to a device meant for filtering, with no direct standalone meaning in this formation.
Воздухозаборный фильтр автомобиля нужно заменить.
The air intake filter of the car needs to be replaced.
In this sentence, part of the word includes 'тель', appearing in the borrowed term 'телеграф', but in modern Russian vocabulary, it's part of the root word rather than a separable morpheme.