Brahman put a straw before him, saying: 'Burn this.' He went towards it with all his might, but he could not burn it. Then he returned thence and said: 'I could not find out what sprite this is.'

In plain words: Brahman put a single straw in front of Agni and told him to burn it. Agni tried with all his strength but could not, so he went back and said he could not find out what the spirit was.