There is much controversy in the Church regarding Speaking in Tongues. What I will do here is explain Speaking in Tongues in as short a version as I can. 

Speaking in Tongues is flowing in the Power of the Holy Spirit. This Power is being poured out incessantly from Heaven by the Lord. 

Another way to put it is, Speaking in Tongues is God Speaking through believers.  

Therefore, being by the right hand of God exalted and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear. (Acts 2:33) 

The purpose for such outpouring is to strengthen the believer and empower the Church. 

A person must repent (turn from) a life of sin and recognized Jesus Christ as the Resurrected Son of God, ask Him to be and accept Him as his Savior and be water baptized to move into this Supernatural activity. (See Romans 10:9) 

When a person receives Jesus Christ as his Savior the Spirit of God (the Holy Spirit) moves into his heart bringing with him his foundational attributes. These are love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, and temperance. (See Galatians chapter 5). 

After having received salvation as outlined above a person becomes a candidate for the baptism in (some say with) the Holy Spirit with the evidence of Speaking in Tongues. 

Now comes that Baptism. This happens at the Hands of Jesus Christ Himself as verbalized by John the Baptist: 

I indeed baptize you with water unto repentance. but he that cometh after me is mightier than I, whose shoes I am not worthy to bear He shall baptize you with the Holy Ghost, and with fire. (Matthew 3:11). 

Baptism with the Spirit at the Hands of Jesus Christ will bring about the Speaking in Tongues by the believer.  

And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. (Acts 2:4). 

Speaking in Tongues is a segue or the threshold to the Spiritual Gifts. The Spiritual Gifts is The Lord performing Supernatural feats co-laboring with the believers (the Church). 

In the Old Testament the Spirit of the Lord would come upon men as God worked miracles through them. That same phenomenon is at work today riding on the heels of Speaking in Tongues.  

But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth. (Acts 1:8). 

UTTERANCE, Speaking in Tongues and Spiritual Gifts