If the side income is occasional and not a sustained business activity, it's often reported as 'Income from Other Sources.' If it's more regular and business-like — ongoing freelance clients, a consistent side business — it's usually classified as 'Profits and Gains from Business or Profession,' which allows you to deduct legitimate business expenses (equipment, software subscriptions, a portion of internet costs) before arriving at taxable income, something salary income doesn't allow.
Under either classification, this income adds to your total taxable income and is taxed at your slab rate on top of your salary — there's no separate lower rate just because it's 'side' income. If your side income crosses certain thresholds, you may also need to pay advance tax through the year rather than settling everything at filing time, since your employer's TDS on salary won't account for this additional income.
Keeping basic records — invoices, payment receipts, and expense proof — from the start makes filing considerably easier than trying to reconstruct a year's worth of side income after the fact.