Learn Russian: Мальчик подбежал к администратору.

The base verb is бежать (to run). Adding the prefix под- gives it the specific meaning of approaching or coming up close to a target. As you are learning in this lesson, verbs of motion with the под- prefix almost always pair with the preposition к (to/towards).

Is подбежал perfective or imperfective?

It is perfective, meaning it describes a single, completed action in the past (he ran up and reached the receptionist). Adding a prefix to a one-way motion verb like бежать automatically creates a perfective verb. If you wanted to say he was "running up" (focusing on the process) or did it regularly, you would use the imperfective form подбегать.

Why does the word for receptionist end in -у (администратору)?

It is in the Dative case. The preposition к always forces the following noun into the Dative case. Because администратор is a masculine noun ending in a consonant, it takes the standard masculine Dative -у ending.

Does администратор always mean "receptionist"?

It is used much more broadly in Russian than the English word "administrator." Depending on the venue, an администратор can be a hotel receptionist, a restaurant host, a clinic's front-desk worker, or a general floor manager who helps customers.