To support or create something. To be a part of the public process that stands up for conserving and protecting things. There is a lot of individual environmental activism. Environmental activists can be found in a range of issue categories, and the politics vary by category too. Especially at the local level, it's involvement in land use decisions that impact sensitive habitats.
Many environmentalists work to save remote places from mining, logging or other forms of intrusions.
Others are working in urban areas trying to deal with toxic pollution and hazardous waste and the legacy of dirty sites. And there are state and local equivalents. Most environmental activists spend time reviewing project documents and responding to the issues that are being brought up - or are not brought up. In California, the California Environmental Quality Act, known as CEQA, requires disclosure of environmental impacts and analysis of alternatives prior to project permitting. Since San Diego has a track record of attempting to get away with inadequate action with respect to CEQA, this can be important work.