Sangha is a Sanskrit and Pali word meaning "community" or "assembly". In early Buddhism it referred to the order of monks and nuns. Today it is often used more widely for any group of people who practice together.
It is one of the Three Jewels of Buddhism, alongside the Buddha and the Dharma, the teaching. The inclusion says something: practice has always been understood as something people support one another in.
For someone practicing alone at home, a sangha might be a local sitting group, an online group or a single friend who also meditates.