icons

You can define custom icons the next way

"Zeioth/heirline-components.nvim",
opts = {
  icons = {
    -- Heirline-components - tabline
    TabClose = "󰅙",
    BufferClose = "󰅖",
    ArrowLeft = "",
    ArrowRight = "",
    FileModified = "",
    FileReadOnly = "",

    -- Heirline-components - winbar
    CompilerPlay = "",
    CompilerStop = "",
    CompilerRedo = "",
    NeoTree = "",
    Aerial = "",
    ZenMode = "󰰶",
    BufWrite = "",
    BufWriteAll = "",
    Ellipsis = "…",
    BreadcrumbSeparator = "",

    -- Heirline-components - statuscolumn
    FoldClosed = "",
    FoldOpened = "",
    FoldSeparator = " ",

    -- Heirline-components - statusline components
    ActiveLSP = "",
    ActiveTS = "",
    Environment = "",
    DiagnosticError = "",
    DiagnosticHint = "󰌵",
    DiagnosticInfo = "󰋼",
    DiagnosticWarn = "",
    LSPLoading1 = "",
    LSPLoading2 = "󰀚",
    LSPLoading3 = "",
    SearchCount = "",
    MacroRecording = "",
    ToggleResults = "󰑮",

    -- Heirline-components - misc
    Paste = "󰅌",
    PathSeparator = "",

    -- Git
    GitBranch = "",
    GitAdd = "",
    GitChange = "",
    GitDelete = "",
  }
},

Note that the default icons we ship require you to use a font from nerd fonts on your terminal.