High level and low level are relative terms so the usage has changed over time. In the 70s UNIX made waves because it showed that an operating system could be written primarily in a high level language: C. At the time C was considered high level as in contrast to assembler. 

Nowadays C is considered a low level language because neither the language nor the standard libraries provide any of the bread and butter data structures like vectors, dictionaries, iterators, and so on. You can have all those structures in a C program, but you'll often end up writing them yourself. Python, Java, etc. are high level relative to C because many of those standard data structures are built in to the language or are part of the standard libraries. Having those right out of the box makes it easier to program at a more abstract level. 

C is low level in a 2nd sense: it enables direct manipulation of the computer hardware (at least as direct as the OS will allow). The most common implementations of Python, Java, etc. are at least one step further removed from the hardware because they run in a VM. If you want to manipulate the hardware from Python you'll have write an extension to the Python VM, usually in C or C++. 

C++ is an odd case. It provides tons of nice data structures as part of the standard library, but it also alows low-level manipulation of the hardware.