Hey, no problem.
An endowment is basically a trust fund for a nonprofit. It is generally invested in stocks, bonds, etc. and managed by the trustees. Each year, income generated from the endowment (around 3-5% on average) is then spent on the organization to keep it running. The larger the endowment, the more money is generated to be used as income.
Most large nonprofits have huge endowments - for example I believe Harvard's is the largest in the country and is $37 billion. You wil see lots of nonprofits that have more money in income than expenditures because of fundraising efforts. Most large gifts made to charities (think $10,000 to $100,000,000) are specifically for the endowment and can't be used in other ways.