If one inning is completed and second is also had few overs usually more than 15 then result is possible.
But in case first inning in progress no chance of result.
Duckworth-Lewis-Stern (DLS) method is much more complicated though but a basic thing is the second innings must complete 20 or more overs to get a result in ODI match.
Let me give you an example, if AUS vs SA is playing an ODI.
AUS batted first for the entire 50 overs and scored 300 runs. Now SA is batting and
Scenario 1: rain interrupted while they only completed 15 overs and No more play is possible means the match will be abandoned and both teams will share each point.
Scenario 2: Field is clear to play after rain stops but with reduced overs let's say 10 more overs than total of 25 overs and the target will be revised based on DLS method which depends on number of wickets in hands which could be 180 if they have 10 wickets in hand or 230 if they have 4 wickets in hand.(yes, it's unfair)
Scenario 3: Rain stops the play at 21st over and no more play is possible now DLS method will apply and if SA is over that target needed at that time then they will be announced as winner and taking 2 points, if not AUS will be announced as winning and 2 points will be given to them.