Summary: Bonus delegates are allocated to locations holding their 2020 contest in April or later. This increases the number of total delegates needed to win the Democratic nomination.
The Democratic Party uses bonus delegates to reward any location* that schedules its nominating contest later in the calendar. A 10% bonus goes to states that will hold their contest in April, 2020, with 20% awarded to those that wait until May or June. Separately, states that hold their primary after March 24th can get a 15% bonus for participating in a 'regional cluster' - basically holding their event on the same date as at least two neighboring states (as defined by the party rules).
The bonuses are additive. For example, if a state holds its primary in April and participates in a regional cluster it would receive a 25% bonus (10% + 15%). The bonus percentage is then multiplied by the Total Base Delegates** that the state was originally allocated to get the bonus delegates.
These additional delegates are pledged, meaning they will be able to vote on the first ballot at the Democratic Convention. In 2020, there will be 211 bonus delegates across 24 locations. Over half of these will be allocated to the six east coast states holding a regional primary on April 28th.
As a result of the bonus delegates, the total number of pledged delegates is increasing to 3,979 from 3,768. This increases the 'magic number' to win on the first ballot from 1,885 to 1,991 Adding 771 superdelegate votes brings the total to 4,750, a majority of which is 2,375.5. (See how these numbers are calculated).