A variant is a strain of SARS-CoV-2 with one or more mutations to its viral genome (genetic code).
Deeper dive
At the center of the SARS-CoV-2 virus is a strand of 30,000 letters [which represent chemical properties] that make up its genome. To reproduce, the coronavirus, via a spike on its membrane, binds itself to the outside of a human cell and then enters it, hijacking the cell’s original genome, directing it to make copies of the SARS-CoV-2 virus instead.
Each time the SARS-CoV-2 virus reproduces, there is a natural possibility for a copying error in the letters, resulting in new variants of the virus. Most of the time, the copying errors are inconsequential or even weaken the ability of the virus to replicate. As the virus fights for survival, however, sometimes it mutates enough to make the pathogen more transmissible and potentially more lethal.
From the beginning of the pandemic, public health officials have been monitoring the genomic sequences of SARS-CoV-2 for changes. In late 2020, variants began to emerge, prompting the CDC to label variants of interest, variants of concern and variants of high consequence, referring to their contagiousness and potential to evade vaccines and treatment. See this World Health Organization explanation of how variants are tracked and named.