Answer by taking a quote from the following article:

Grace Brewster Murray Hopper (nee Murray; December 9, 1906 - January 1, 1992) was an American computer scientist and United States Navy rear admiral. One of the first programmers of the Harvard Mark I computer, she was a pioneer of computer programming who invented one of the first compiler related tools. She popularized the idea of machine-independent programming languages, which led to the development of COBOL, an early high-level programming language still in use today. Hopper had attempted to enlist in the Navy during World War II, but she was rejected by the military because she was 34 years of age and thus too old to enlist.

In 1949, Hopper became an employee of the Eckert-Mauchly Computer Corporation as a senior mathematician and joined the team developing the UNIVAC I. Hopper also served as UNIVAC director of Automatic Programming Development for Remington Rand. The UNIVAC was the first known large-scale electronic computer to be on the market in 1950, and was more competitive at processing information than the Mark I.  When Hopper recommended the development of a new programming language that would use entirely English words, she "was told very quickly that [she] couldn't do this because computers didn't understand English." Her idea was not accepted for 3 years, and she published her first paper on the subject, compilers, in 1952. In the early 1950s, the company was taken over by the Remington Rand corporation, and it was while she was working for them that her original compiler work was done. The program was known as the A compiler and its first version was A-0.  In 1952 she had an operational link-loader, which at the time was referred to as a compiler. She later said that "Nobody believed that," and that she "had a running compiler and nobody would touch it. They told me computers could only do arithmetic." She goes on to say that her compiler "translated mathematical notation into machine code. Manipulating symbols was fine for mathematicians but it was no good for data processors who were not symbol manipulators. Very few people are really symbol manipulators. If they are they become professional mathematicians, not data processors. It's much easier for most people to write an English statement than it is to use symbols. So I decided data processors ought to be able to write their programs in English, and the computers would translate them into machine code. That was the beginning of COBOL, a computer language for data processors. I could say "Subtract income tax from pay" instead of trying to write that in octal code or using all kinds of symbols. COBOL is the major language used today in data processing."  In 1954 Hopper was named the company's first director of automatic programming, and her department released some of the first compiler-based programming languages, including MATH-MATIC and FLOW-MATIC.

what made them better
first known large-scale electronic computer to be on the market