Your excess is the amount you need to pay when you want to make a claim. For example, you might be paying $100 a month for home insurance, when your home is damaged in a fire. It is estimated to cost $10,000 to make the necessary repairs. According to your policy, you might need to pay $500 towards this claim - that is an excess (it is usually taken from the payout amount).
However, if you want to reduce your premium from $100 a month to $75 a month, you might be able to increase your excess. Whether you can increase your excess and the amount varies from insurer to insurer. It could also mean that you're less likely to make claims for small amounts. There's little point making a claim to replace a $300 window if the excess is $800 (you'll need to pay $800 to get $300 back).