What is the difference between Lead and Lag functions

The Lead function returns values from the next row, while the Lag function returns values from a previous row.

Example

Select start_date, 
lead (start_date,1) over (ORDER BY start_date) AS next_start_date
lag (start_date,1) over (ORDER BY start_date) AS prev_start_date
From contracts
Where customer_id = 760;