BASIC is a high level* programming language that used to be very popular especially with the first few generations of home computers in the 1980's to the 1990's. Many of the computers would boot to the BASIC prompt in the same way an old pre-Windows PC would boot to the DOS prompt.
BASIC stands for Beginners All-Purpose Symbolic Instruction Code and was developed in the 1960's as an easy to understand programming language that used simple code such as " Print "Hello" " and FOR...NEXT loops. A great many versions of BASIC were available, from very simplistic versions such as available on Commodore 8-bit computers, to more sophisticated versions such as BBC BASIC on the BBC Micro.
BASIC survives to day as Visual BASIC and a number of specialised variants, like "ThinBASIC" but it has been surpassed by the various versions of C and, for IF, the specialised authoring systems like TADS and Inform.
BASIC, generally speaking, did not provide much in the way of speed as it was an Interpreted language. Compiled languages require the program to be written, then compiled to create a runtime program which is then executed. Interpreted languages provide a source code file which is compiled line by line as a line is required. So every time a line of BASIC is run, it is recompiled in realtime.
High level - generally closer to human understandable language, Inform7 is very high level. Low level languages are closer to the binary that computers understand, such as Assembly Language.
LET A= 5 as opposed to LDA 5 in assembler.