What does the verse mean to you? “To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.” (Isaiah 8:20)
To understand such verses, it’s important to understand to “whom” and “when” those verses were spoken.
In this case, it was spoken during the OT Time. Hence the “law” was to be practiced (religious rituals).
But the second part is true till today because it’s a general statement, “if they speak not according to this word, it is because there is no light in them.”
Why? Because the first part of “law & its testimony” today means, the “NT law” and “its testimony” as well as need to be according to this New Covenant’s “Word”.
“Word” = “Let no one, then, judge you in eating or in drinking, or in respect of a feast, or of a new moon, or of sabbaths,” (Colossians 2:16)
“NT Law” = No judging others according to the ritualistic laws of the OT Covenant with literal examples mentioned as Sabbath, Feasts, New Moon, food & drink prohibition etc.
“its Testimony” = No NT verses (after Christ has Resurrected and after the OT Covenant has been abolished) ever states the observance of any of those ritualistic OT Laws (such as tithes, Sabbath, circumcision etc) by the earliest believers in the Bible except (“to abstain from things offered to idols, and blood, and a strangled thing, and whoredom; from which keeping yourselves, ye shall do well; be strong!“)– Acts 15:29:
1. Do not eat strangled animals.
2. Do not eat food offered to idols.
3. Do not drink blood
The other one, keep away from “sexual immorality” (whoredom = prostitution with or without money = both having sexual relations with a man/woman outside marriage is wrong , note: marriage in the Bible is literally between a man & a woman only) is a non-ritualistic command – All non-ritualistic sin is still sin under the NT Covenant – as other verses also prove this – plus without holiness (missing the mark, sinning), no one can see the Lord. Be holy for I am Holy (always true) regardless of Covenant since it’s regarding God Who does not change from this Good.
To ignore means there is no “light” (especially of knowledge & wisdom) in them.
Anything more is just adding to His Word (literally).