Everybody listening to Jesus would have understood about sowing seed by hand. It was not an accurate process to ensure that the seed only landed on fertile soil; some seed would go on the path. Also, the sower would not be alone. Birds would flock to the field in the hope of a meal. It would be more difficult for them to find the seeds in the crevices of the ploughed soil, but whatever landed on the path would be 'easy pickings'.
 
In Jesus' parable, the seed represents the gospel; telling people how to get ready to be a part of God's kingdom. That message is from God (God's Word), but scattered by His messengers (the apostles and later, the church). The soil represents the hearts of the hearers. In this case the path. Paths are really just tracks of soil which had become hard through being trodden down and made unreceptive to any seed.
 
The trainee apostles needed to know that they are not alone. Yes, they may be working in a team or in pairs, and they had God's Holy Spirit. But the Evil One is there too. Satan, is the personal name of the devil - he is evil in every way, although he pretends to be an angel of light (2 Corinthians 11:14). Not only does he work by telling lies (John 8:44). But he also tries hard to snatch away the Word, to stop the person thinking about it or allowing it to sink into the heart. Satan does this in lots of ways: distracting, confusing, ridiculing and frightening us. His aim is to stop us understanding its meaning and therefore to ignore what God is saying.
 
This world is not spiritually neutral. It is in the grip of the 'prince of this world', Satan, the pretender to Jesus' throne (John 12:31). The gospel is the key to releasing people from his tyranny and is therefore 'dangerous' to him. So the Evil One tries to deny people access to that truth. He will encourage their hearts to be hard against the Lord, and will blame the hard circumstances of life on a 'god who does not care'. Satan will do everything he can to build up resistance to God's Word. Yet, in the same way that some seed will lodge in a dusty crack, if it is given time to take root it has the power to open the path. Some evangelism is like that.