Here is a reference to the various levels put forth by the Common European Reference Framework for Languages. The following interpretations are mine.
A-1 Knows only the most basic phrases and expressions. Can converse after a fashion on a handful of topics.
A-2. Has a limited command of vocabulary and expressions that goes beyond "basic." Can function in a broader range of situations than an A-1. Can sustain a conversation, or write a passage, of modest length and depth.
B-1. Has a level of proficiency that is good enough for "familiar," ordinary, or everyday situations. Can "get along" in the language, although with some difficulty.
B-2. Beginning to exhibit proficiency or mastery in complex topics, particularly those related to one's interests or profession. Can handle most situations without difficulty. Mistakes and "accent" are starting to "go away."
C-1. Has a deep understanding of both the vocabulary and the structure of the language, and can use the language fluently. Is able to express oneself clearly and accurately using complex constructions in many situations. Makes only occasional mistakes.
C-2 Proficient across a wide range of topics, almost, but not quite as fluent as a well-educated native speaker. Mistakes are rare, almost non-existent.
In US State Department terms, someone with a B2/C1 level has "professional" proficiency, and someone with a C2 level has "full professional" proficiency.