Gene transcription is controlled (turned on or off) by regulatory elements that are part of the gene. These regulatory elements are also called cis-regulatory elements and they are usually, but not always, located on the 5' side (also called upstream) of where the transcription of the gene starts. These cis-regulatory elements serve as recognition and binding sites for transcription factors. Transcription factors are proteins and, as their name implies, they regulate gene transcription. When a transcription factor binds to a regulatory element present in a gene it will recruit other proteins, including RNA polymerases that will allow transcription to occur. Genes often have more than one type of regulatory element, and many have several copies of the same regulatory element.
UAS is an abbreviation for DNA regulatory sequences known as "Upstream Activating Sequences". GAL1 has several UAS elements and deleting one of them will diminish the binding of transcription factors that enhance the transcription of GAL1. If there is less transcription factor bound there will be less transcription of that gene (in this case GAL1).